This application is based on and claims priority of Japanese Patent Application No. 2000-66170 filed on Mar. 10, 2000, the content of which is incorporated herein by reference.
1. Field of the Invention
This invention relates to a microscopic optical system for forming an intermediate image of a specimen by an objective lens and imaging the intermediate image again by a relay optical system, and more particularly to a microscopic optical system which has good off-axis performance, such as chromatic aberration of magnification, when using light in an ultraviolet range, especially of short wavelengths, and which is suitable for image pickup by a CCD (charge-coupled device) camera or the like.
2. Description of Related Art
Recently, in the semiconductor market, as IC patterns are refined, microscopes for observing them are increasingly required to have high resolution. One of the techniques to realize a microscope of high resolution is to shorten the wavelength of its light source. The wavelength is inversely proportional to the resolution. If the wavelength becomes half, the resolution doubles.
Accordingly, the development of microscopes using ultraviolet rays is expected.
Examples of lenses for ultraviolet rays used in optical systems for ultraviolet rays are described in Japanese Laid-Open Patent Publications Nos. Hei 8-179200 and Hei 10-260349.
Japanese Laid-Open Patent Publication No. Hei 8-179200 describes a collimating lens for ultraviolet rays which uses only quartz as its glass material.
Japanese Laid-Open Patent Publication No. Hei 10-260349 describes an imaging optical system for an ultraviolet laser beam which uses quartz and fluorite as its glass materials.
In constructing an ultraviolet microscope, if a laser light source is used as its ultraviolet light source, the light quantity of the light source is sufficient, but its lifetime is short and it is expensive. Accordingly, its running costs are high, which is a problem.
On the other hand, if an arc light source, such as a mercury lamp, is used, its running costs can be kept low. However, if it is used with a single wavelength, no infficient luminance can be obtained so that only dark observation can be made. If it is used with a wide range of wavelengths, bright observation is possible. In this case, however, if chromatic aberration of the optical system, such as on-axis chromatic aberration and chromatic aberration of magnification, is not corrected, its imaging performance deteriorates so that no sufficiently high resolution can be obtained. That is, if the on-axis chromatic aberration of the optical system is not corrected, the focal position differs for each wavelength. Thus, when observation is made with a CCD camera or the like (since the ultraviolet rays cannot be observed with the naked eye), in-focus and out-of-focus images are mixedly picked up so that the resultant image of poor imaging quality only can be observed. Further, if its chromatic aberration of magnification is not corrected although the on-axis chromatic aberration of the optical system is corrected, images of respective wavelengths are in focus, but the magnifications of the images of respective wavelengths are different. Accordingly, on the peripheries of the respective images, their imaging positions are different so that the images appear to be displaced laterally. Thus, the respective images displaced laterally are superposedly picked up by the CCD camera and cannot be distinguished from each other so that the resultant image appears as if a point image were expanded. Consequently, on its periphery, the resultant image of poor image quality only can be observed.
Moreover, in view of the use of the CCD camera, the angle of off-axis rays also needs to be considered. When the CCD camera is used for image pickup, it is the most preferable that light is incident on the light receiving surface of the CCD perpendicularly (at 90xc2x0). If the angle of light incident on the light receiving surface differs largely from 90xc2x0, the quantity of light is lost due to the sensitivity characteristic of the image pickup device (CCD) with respect to the angle of light incident on its light receiving surface (shading phenomenon) so that the periphery of the image is darker than its center. Accordingly, the angle of off-axis rays (position of the exit pupil) must be considered at the time of lens design.
Since the optical system described in Japanese Laid-Open Patent Publication No. Hei 8-179200 comprises a refractive lens whose glass material is quartz only, chromatic aberration cannot be corrected. Thus, even if this conventional optical system is simply developed, it is impossible to attain a microscopic optical system for a range of ultraviolet wavelengths which has good imaging performance. Further, in this conventional optical system, the image flatness is considered, but the position of the exit pupil is not referred to. Accordingly, even if this conventional art is simply developed to construct a relay optical system, it cannot cope with the shading phenomenon of the CCD camera.
The conventional art described in Japanese Laid-Open Patent Publication No. Hei 10-260349 uses fluorite for a convex lens and quartz for a concave lens and is developed as an optical system for an ArF excimer laser.
Since the ArF excimer laser has several oscillation lines within a wavelength range of about 1 nm according to its construction, it is preferable that chromatic aberrations are corrected within this wavelength range. Thus, in order to correct on-axis chromatic aberration, fluorite is used for the convex lens and quartz for the concave lens. However, even if this technique is straightforwardly developed, it is impossible to reach a microscopic optical system in which chromatic aberration of magnification is also considered.
Moreover, this conventional art does not refer to the position of the exit pupil and the like. Even if this art is developed, one cannot obtain a relay optical system which can eliminate the shading phenomenon at the time of image pickup by the CCD camera.
An object of the present invention is to provide a microscopic optical system for forming an intermediate image of a specimen by an objective optical system and imaging the intermediate image again by a relay optical system, the microscopic optical system using light in an ultraviolet range, especially of short wavelengths, and having good off-axis performance, such as chromatic aberration of magnification.
A microscopic optical system according to the present invention comprises an objective optical system for forming an intermediate image of a specimen and a relay optical system for relaying the intermediate image, wherein the relay optical system comprises at lease one quartz lens having positive refractive power.